1. Field of the Invention
The invention relates to surface-modified pyrogenic silicas which are suitable as Theological additives in liquid media containing strongly basic groups, to the use of the silicas of the invention, and to their preparation.
2. Background Art
Pyrogenic (or fumed) silicas are widespread additives for controlling the flow properties of adhesives, sealants, and coating materials such as inks or paints. Pyrogenic silicas are used to adjust the viscosity, to adjust the shear-thinning and thixotropic properties, and to adjust the yield point.
For apolar systems of relatively high molecular weight with a high solvent fraction, it is preferred to use nonmodified, i.e., hydrophilic pyrogenic silicas. Surface-modified, i.e., hydrophobic silicas, such as silicas modified with dimethylsiloxy groups as described in DE 1163784, are preferably used in polar systems of low molecular weight with a low solvent content, or in solvent-free systems.
Establishing a viscosity or yield point that is stable over a prolonged period for liquids comprising components containing strongly basic groups, such as amine groups, has proven difficult. Driven by an acid-base reaction between the acidic silanol groups of the silica and the basic amine groups, hydrophilic silicas exhibit irreversible adsorption of the aminic component and hence deficient long-term stability of viscosity and yield point. Hydrophobic silicas, such as those modified with dimethylsiloxy groups, likewise exhibit inadequate long-term stability of viscosity and yield point.